Drop ggplot2 dependency

* R/plot.r and R/punch_card.r: Delete from git2r package. Move plot
  functionality to the ggit package.

* man/plot-methods.Rd and man/punch_card-methods.Rd: Delete

* DESCRIPTION: Update

* NAMESPACE: Update

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>